The present invention relates to a non-dusty investment material compounded with liquids and containing water-soluble phosphates, magnesium oxide and silicon dioxide, for the production of accurately fitting cast parts in dentistry and in the jewelry industry, and to a method of producing such investment materials.
In dentistry and in the jewelry industry, metallic dental prosthesis parts and jewelry parts are generally produced by means of casting methods. To this end, the dental prosthesis part or jewelry part is modeled in wax, embedded, the wax removed by melting, and the molten alloy is then poured into the mold formed in this manner.
High requirements, with respect to the accuracy of the fit of the parts produced from these masses, are placed on the investment materials used for the production of the molds. Thus the contraction of the cast metal part due to the cooling off after the casting must be exactly compensated by the setting expansion and the thermal expansion of the investment material.
Three different types of investment materials are used in dentistry which differ by virtue of the binder system used:
a) Phosphate-bound investment materials; PA0 b) Gypsum-bound investment materials; and PA0 c) Silicate-bound investment materials.
The most widespread are the phosphate-bound investment materials containing magnesium oxide and silicon dioxide since they are highly temperature resistant and can also serve as molds for high-melting burned-on alloys. Phosphate-bound investment materials are stirred with water or a water/silica sol mixture. The addition of the water brings about the setting reaction of the two binder components ammonium phosphate and magnesium oxide.
Silicon dioxide in the form of quartz and its modifications is used as refractory components in all three types of investment materials. In order to obtain a smooth cast surface, the refractory components must also contain very fine particle sizes. Therefore, a development of dust always occurs during the processing (e.g., filling in, weighing, stirring) of the investment materials. The dust which evolves during processing represents a considerable health hazard for the dental technician and the goldsmith. The breathing in of quartz-containing dust can result in silicosis. The respirable fine components of the dust are especially dangerous in this respect.
For this reason, lawmakers have set maximum concentrations of dustiness in the workplace, for example, with the dangerous substances regulation of Apr. 26, 1986 in Germany. The currently valid limit values are about 6 Mg/M.sup.3 total dustiness or 4 mg/M.sup.3 for quartz-containing fine dust and 0.15 Mg/M.sup.3 for respirable, quartz-containing fine dust.
Given the current use of customary investment materials, manufacturers can only remain below these values in a permanent and reliable manner if the work areas are equipped with complicated and expensive suction removal and filter systems.
DE patent 37 07 853 describes a powdery investment material which exhibits only a slight dust formation. This is achieved in that the powder mixture of soluble phosphate, magnesium oxide and quartz is supplied with 0.5-5% of a wetting agent consisting of liquid, hydrophobic hydrocarbons, fatty acid esters or fatty acids. Since these liquids exhibit a low vapor pressure, they are always relatively long-chain, organic compounds which result in an oily feel in the case of the investment materials and reduce the kneadability, so that anionic, surface-active means must also be added. However, this can negatively alter the technical properties of the investment materials (e.g., low strengths, too high setting expansions, unpleasant odor).
The addition of a liquid to the investment material can also take place for other reasons. Derwent abstract 84-003437/01 describes an investment material to which an aliphatic alcohol is added in amounts of 0.01 to 0.3% by weight in order to prevent the reaction of the binder material on account of the atmospheric humidity and to increase the storage stability therewith. However, the amounts added are so small that no freedom from dust is obtained in this manner.